That sounds like a good option that should not be too hard to implement down the road. Of course, I'm not a programmer and some from ooma might find it more difficult than this armchair coach.

My main question is why this would be a problem for people using your rental? Do you allow them to make International calls on the system? This strikes me as a very generous situation of that is the case. Normal calls (anywhere in the US) will never deplete your balance so most people would not find it a problem.